The Hohmann transfer orbit is the most propellant-saving transfer orbit to Mars. When using this kind of transfer, the rover/rocket/etc. starts from Earth when Earth and Mars are in conjunction.

But now, in 2020, many Mars missions will be launched, but Earth and Mars are in opposition. I do not understand why they do that, since the Hohmann transfer orbit saves the most propellant?

If somebody would explain me the reasons, I would be very glad!